Thermal printheads have been widely used for printers of office automation machines such as facsimile machines, for printers of ticket vending machines and for label printers. As is commonly known, the thermal printhead selectively provides heat to a printing medium such as thermosensitive paper or thermal-transfer ink ribbon to form needed image information.
For the convenience of explanation, the structure of a typical prior art thick film-type thermal printhead is shown in FIG. 12 of the accompanying drawings. The thermal printhead 100 shown in that figure comprises a solid insulating substrate 102 made of e.g. a ceramic material and fixed on a heat sink plate 101 which is made of a metal such as aluminum having a high thermal conductivity. The upper surface of the substrate 102 is formed with a glass glaze layer 103 as a heat retention member, and a heating resistor 104 is formed linearly on top of the glaze layer 103 to provide an array of heating dots. Further, the upper surface of the substrate 102 is provided with a plurality of drive ICs 105 for supplying electric power to the heating resistor 104.
Further, the substrate 102 carries a common electrode 106 extending on the glaze layer 103 and having comb-like teeth in conduction with the heating resistor 104, and a plurality of individual electrodes 107 similarly extending on the glaze layer 103 and electrically connected to the heating resistor 104. The individual electrodes 107 are connected to the drive ICs 105 via bonding wires 108. The heating resistor 104, the common electrode 106 and the individual electrodes 107 are covered with a protective layer 109 made of a glass material for example.
In the thermal printhead having the above-described arrangement, a predetermined voltage is selectively supplied from the drive ICs 105 via the individual electrodes 107 while the voltage of the common electrode 106 is kept at a constant value, thereby selectively actuating the heating dots of the heating resistor 104 to thermally form images on thermosensitive paper for example.
Generally, it is necessary to enhance the heat retaining ability near the heating resistor 104 in order to improve the printing performance with a small electrical power. For this purpose, in the prior art thermal printhead described above, the glaze layer 103 formed below the heating resistor 104 provides a heat retaining function. On the other hand, a portion of the heat from the heating resistor 104 which has already escaped to the substrate 102 is no longer utilizable for printing. Thus, the heat sink plate 101 serves to quickly dissipate the escaped heat into the atmosphere for preventing a temperature rise of the substrate 102 as a whole.
However, in the prior art thermal printhead described above, since the glaze layer 103 alone cannot provide a sufficient heat retaining function, the amount of heat which is dissipated into the atmosphere via the substrate 102 and the heat sink plate 101 will increase, consequently failing to provide a satisfactory printing performance when the supplied power is reduced below a predetermined level.
On the other hand, with a recent development of various office automation machines, there is an increasing demand for portable thermal printers of battery drive type (i.e., low power consumption type). However, the above-described prior art thermal printhead is not suitable for constituting a portable thermal printer of battery drive type (i.e., low power consumption type).
In order to overcome such a problem, Japanese Patent Publication No. 3(1991)-21352 for example proposes formation of a hollow portion 110 (shown by the phantom lines in FIG. 9) in the glaze layer 103 to additionally increase heat retention near the heating resistor 104. The hollow portion 110 may be formed, for example, by the steps of forming a strip-like dissolvable layer (of e.g. silver) on the substrate 102, forming a glaze layer 103 to cover the dissolvable layer, and then dissolving the dissolvable layer with a chemical solution.
However, the above-described solution has a cost increase problem because the steps of forming the hollow portion 110 (formation and removal of the dissolvable layer) are troublesome.
Further, since the formation of the hollow portion 110 needs the presence of the glaze layer 103 as a prerequisite, it is impossible to form such a hollow portion 110 if the substrate 102 itself is formed of a material having a low thermal conductivity for power consumption decrease to obviate the need for a glaze layer 103. Moreover, since the size and like of the hollow portion 110 are restricted by the thickness and like of the glaze layer 103 for example, the range of realizable printing performance is also greatly limited.